Various devices for internal fixation of bone segments in the human or animal body are known in the art. One type of system is a pedicle screw system, which is sometimes used as an adjunct to spinal fusion surgery, and which provides a means of gripping a spinal segment. A conventional pedicle screw system comprises a pedicle screw and a rod-receiving device. The pedicle screw includes an externally threaded stem and a head portion. The rod-receiving device couples to the head portion of the pedicle screw and receives a rod (commonly referred to as a distraction rod). Two such systems are inserted into respective vertebrae and adjusted to distract and/or stabilize a spinal column, for instance during an operation to correct a herniated disk. The pedicle screw does not, by itself, fixate the spinal segment, but instead operates as an anchor point to receive the rod-receiving device, which in turn receives the rod. One goal of such a system is to substantially reduce and/or prevent relative motion between the spinal segments that are being fused.
Prior art pedicle screw systems often include a top-loaded set screw compression member that is threaded into a coupling member that receives the pedicle screw and rod. It is often difficult for the surgeon to reposition or adjust the spinal rod relative to the fixation system during the surgical procedure after the compression member is inserted into the cap. Once the compression member is initially threaded into the coupling member, any space between the spinal rod and the compression member for final positioning of the rod prior to locking is randomly achieved. For example, the surgeon can randomly thread the compression member a few turns to provide a re-positioning gap between the compression member and rod or completely thread the compression member into a locking position and then back-off the threading to form this re-positioning gap. As a result, the current threaded systems for immobilizing vertebral bones with spinal rods typically require the surgeon to spend more time and guess work for achieving a gap between the compression member and rod for any final positioning of the rod prior to locking. If the re-positioning gap is not large enough, the spinal rod may bind during repositioning, thereby requiring even additional time and adjustment of the compression member to form a larger gap. All this random threading and guesswork by the surgeon requires additional time in the operating room for performing the surgical procedure on the patient.
Spinal fixation systems including locking components that only require axial shifting for locking thereof are known alternatives to utilizing rotatable threaded members. Such as axially locking spinal flexible systems are disclosed in applicants' assignees' U.S. patent application Ser. No. 11/726,868, as well as U.S. Provisional Applications Nos. 60/784,674 and 60/981,821. These systems include an anchor member (e.g., a screw or hook), a compressible inner tulip member that receives a spinal rod and a pedicle screw head snap-fit thereto, a rigid outer tulip that shifts axially over the inner tulip to compress the inner tulip tightly onto the screw head, and a cap member axially inserted between portions of the inner and outer tulip member to compress the inner tulip about the rod. This system is hereinafter referred to as the Low Top™ system, with the reservation that the instruments described herein may also be used with other spinal fixation systems with axially locking components.